Doctor of Philosophy
Mechanical and Nuclear Engineering
Air pollution is a major threat to environmental safety and public health. Volatile organic compounds (VOCs), particulate matter (PM), and airborne microorganisms are three typical air pollutants. Conventional strategies to prevent and mitigate air pollution have been employed, which, however, are generally passive. For instance, VOC sensing through solid-state devices is a conventional approach, which, however, is not capable of capturing and removing VOCs. On the other hand, air filters and face masks are useful equipment to protect people from inhaling PM and airborne microorganisms. But most commercial filters can only capture them on the surfaces, which may cause secondary contamination under high airflow rates. This is particularly true for airborne pathogens, such as bacteria, fungi, and viruses, which can survive on the filter surface for hours or even days, creating a potential risk of biosafety issues. Therefore, it is highly desired to develop advanced materials to solve the above air issues actively.
Metal-organic frameworks (MOFs), a novel porous crystalline material, have received considerable attention over the past decades because of their exceptional physical and chemical properties, including high porosity, huge specific surface area, structure robustness, and chemistry flexibility and diversity. These intriguing properties make MOFs an excellent candidate to combat the above air contamination. Numerous attempts have been reported to improve the performance of MOFs. However, the challenges remain in MOF design and its applications because of the complex interactions between MOFs and target pollutants.
The objective of this dissertation is to rationally design MOFs-based functional materials for efficient air quality control. This dissertation also aims to explore the quantitative interactions between the target air pollutants and the MOF-based materials by using advanced instruments, which generate new knowledge and understanding for future materials designed for better air quality. The dissertation is divided into two major parts: VOC (H2S as a model) detection and airborne bacterial inactivation. In Chapters 2 and 3, a novel bimetallic MOF (i.e., Al/Fe-MIL-53-NH2) was developed to significantly improve its sensing performance towards a representative VOC of H2S molecules based on the fluorescence “turn-on” effect. Beyond the improved performance of MOF-based materials, fundamental understandings of interactions between H2S and MOF-based materials were also discussed. More specifically, the mechanisms of H2S detection were successfully unraveled, where nitro-MOFs (e.g., Al-MIL-53-NO2) were used to achieve quantitative fluorescence sensing. The new insights based on the investigations in this dissertation are completely different from what has been reported in previous studies. The results showed that it is the free BDC-NH2 (2-aminobenzene-1,4-dicarboxylic acid) in the solution rather than the formation of Al-MIL-53-NH2 that caused the fluorescence enhancement.
In Chapters 4 and 5, novel antimicrobial materials have been designed by coating a quaternary ammonium compound (QAC) polymer, that is poly[2-(dimethyl decyl ammonium) ethyl methacrylate] (PQDMAEMA), onto the surface of various MOF-based materials (e.g., UiO-66-NH2 (zirconium-based), g-C3N4/MIL-125-NH2 (titanium-based)) to form active composites for airborne bacterial inactivation. These rationally designed MOF composites demonstrated great antibacterial activities where electrostatic contact-killing and photogenerated reactive oxygen species (ROS) are utilized for efficient disinfection. In-depth investigations on the biointerface were carried out with several advanced techniques, such as the Zeta-potential analyzer, fluorescence laser confocal microscope (CLSM), and atomic force microscope (AFM). The results showed that the adhesion of bacterial cells towards the photocatalyst surface leads to significantly enhanced photocatalytic bactericidal efficiencies.
The work from this dissertation is expected to broaden the applications of MOF-based materials and advance the understanding of the interactions between MOFs and pollutants from the molecular level, which should have a significant impact on the rational design of MOF-based materials for air quality control and improvement.
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